Challenges and Opportunities in the Selective Laser Melting of Biodegradable Metals for Load-Bearing Bone Scaffold Applications

The aim of this paper is to assess the current status of processing biodegradable metals (BDM) via selective laser melting (SLM), with particular emphasis on bone scaffold applications, and provide a meta-analysis on the effect of processing parameters on relative density to better direct recommendations for the future of this growing field. Synthetic bone scaffolds are becoming a popular alternative for the treatment of critical bone defects that cannot heal without surgical intervention. These scaffolds act as a bridge allowing bone to grow across the gap. Selective laser melting can achieve bone scaffolds with complex hierarchical architecture tailored specifically to the patient. SLM-manufactured titanium scaffolds have already been clinically tested with some success. Permanent titanium alloys have a higher chance of implant rejection from the innate immune reaction, coupled with complications linked to the high mismatch in stiffness between the implant and the bone. Biodegradable metals can overcome these problems by maintaining sufficient mechanical properties for load-bearing applications during healing and eventually degrade away completely. Currently, however, the use of SLM for the manufacturing of BDM scaffolds is still in its infancy as only a few peer-reviewed studies are published, with the majority of these published in the last couple of years. Literature was systematically reviewed to critically analyze and synthesize the data in the form of a meta-analysis. Only studies that included the processing parameters used for volumetric energy density (namely the laser power, scan speed, hatch spacing, and energy density) and provided as built relative densities were used. SLM of biodegradable metals is an exciting research area that requires further exploration. Apart from overcoming the problems unique to each major biodegradable metal family, the meta-analysis showed that the vast majority of studies regard the optimization of SLM processing parameters. However, these studies are specific to the powder and machine used. Rather, broader guidelines need to be developed for modern SLM machines to allow for quicker optimization for future SLM-manufactured BDM.